THE FORMAL withdrawal of American forces from Iraq never meant that the United States could categorically disengage from that country. As fighters for the Islamic State in Iraq and Syria advanced toward the previously stable Kurdish-dominated city of Irbil this week, and as the massacre of thousands of members of the Yezidi religious group appeared likely, President Obama’s decision to order US airstrikes against the militant group was the least objectionable option. It need not — and should not — signal a re-escalation of a war whose human toll has been horrific.

Even as he announced his decision late Thursday, Obama made it clear that there were limits to the engagement and that the United States would not get involved in another land war in Iraq. His evident reluctance revealed the airstrikes for what they are: a stopgap measure to keep ISIS in check, while also buying time for a more competent and inclusive Iraqi government to emerge.

The airstrikes, which have been coupled with humanitarian aid for civilians in danger, meet a narrow set of criteria: They were invited by the relevant governing authority, namely Iraqi Prime Minister Nouri al-Maliki; they helped to prevent what, some feared, might become a genocide; and they served a strong US interest — in this case, protecting Iraq’s stable and largely autonomous Kurdish region from outside attackers. Beyond the threat to the Yezidis, the recent advance of ISIS fighters threatened to create a population exodus that would render Irbil and other areas still more vulnerable to attack — and give ISIS access to oil supplies that the militant group could use to enrich itself.

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Obama also had reason to be confident that US involvement would help forestall such an outcome, and at an acceptable cost; during the later years of Saddam Hussein’s regime, enforcement of the so-called no-fly zone in northern Iraq prevented major military operations against Kurdish areas.

The president’s decision to order airstrikes now, thereby shoring up a de facto Kurdish state, contrasts with his more restrained response to the ISIS advance elsewhere. In the Sunni Arab-dominated areas where it has gained power, the group, despite its brutality, enjoys some support from powerful local interests. Indeed, ISIS owes its recent successes in no small part to Maliki’s increasingly sectarian Shiite-led government, which has given Sunnis little reason to believe a unified, pluralistic Iraq is possible.

Obama has no illusions, either. If anything, his decision to assist the Yazidis and the Kurds showed a recognition that Washington’s influence over Iraq as a whole is limited. The US administration should do what it can to encourage Maliki to leave office.

But precisely because it can’t dictate the outcome of the current power struggle, the Obama administration had good reason to react in real time to an emerging humanitarian threat. And having unleashed the forces now struggling for control over the country, the United States has lost any moral justification for looking the other way.